RU2798745C1 - Method for studying the core of terrigenous rocks - Google Patents

Abstract

FIELD: petrophysical research.

SUBSTANCE: invention can be used to determine the conditions for the flow of elastic and irreversible deformations of rocks in drained formations in the development of oil and gas fields and the operation of underground gas storages (UGS). A core of terrigenous rocks is saturated with fluid and placed in a core holder. The core and fluid in the core are brought to reservoir conditions and the deformation properties of rocks are determined with a stepwise change in the initial pressure in the core. At the same time, the deformation properties of rocks are determined with a cyclic stepwise decrease and the same increase in pressure in the core with an increase in this value at each cycle, the core deformation and the amount of pressure reduction in the core are recorded, at which the core deformation, determined with a decrease in pressure, becomes different from the deformation core determined at the same pressure increase in the core. According to the such minimum decrease in pressure, the pressure value in the core is determined, below which the deformation of the rocks of the core passes from elastic to irreversible, and according to the maximum decrease, at which there is no deformation of the rocks at the stage of pressure increase, the pressure is determined, below which the deformation of the rocks becomes only irreversible.

EFFECT: ability to assess the feasibility of constructing geodynamic polygons in UGSs and to determine drawdowns on the reservoir when defining operating conditions of production wells that do not lead to the destruction of reservoir rocks in the bottomhole zone.

1 cl, 1 dwg

Description

The invention relates to the field of petrophysical research and can be used to determine the conditions for the flow of elastic and irreversible deformations of rocks in drained formations in the development of oil and gas fields and the operation of underground gas storages (UGS).

A known method of studying the core, the implementation of which determines the complex of petrophysical properties of a sample of rocks in the simulation of reservoir conditions. In this case, the core is saturated with gas, an all-round pressure is created that exceeds the pore pressure, and the pore pressure is up to the pore pressure in the simulated reservoir. At a constant value of the pore pressure, the all-round pressure is increased stepwise to a value corresponding to the value of the all-round pressure in the formation under study. The studies are repeated after reducing the pore pressure to atmospheric pressure, evacuating and saturating the core with liquid. At each stage of pressure change, the petrophysical properties of the sample are determined: open porosity coefficient, propagation velocity of longitudinal and transverse waves, electrical resistance, gas permeability coefficient (RU 2781413, 2022).

However, when implementing this method, the dynamics of the deformation properties of the studied core is not determined.

A known method for studying the core, which consists in assessing the change in the permeability of rocks under conditions of variable (cyclic) pore pressure and constant vertical and lateral compressive stresses (Indrupsky I.M., Ibragimov I.I., Zakiryanov R.A., Girfanov I.I. Changes in carbonate reservoir permeability under cyclic geomechanical impact, Oil Province, 2020, No. 3(23), pp. 85-98, http://www.vkro-raen.com).

This method is designed to study the dynamics of the filtration parameters of carbonate rocks as a result of a cyclic change in pore pressure in the studied rock sample (core), and the change in deformation properties is not studied.

Closest to the described method is the method of studying the core of terrigenous rocks, in the implementation of which, after placing the core in the core holder, reservoir conditions are simulated. The core is saturated with fluid, axial and compressing pressures are created, then, with a stepwise decrease in pore pressure, the deformation of the studied rock sample is determined (Kuzmin Yu.O., Zhukov V.C. Modern geodynamics and variations in the physical properties of rocks. - M .: MGGU Publishing House, 2004. - 280 p.).

This method allows you to determine the dynamics of deformation of the studied sample of rocks, but the nature of the deformation is not determined.

The purpose of the invention is to determine the magnitude of the change in reservoir pressure in the simulated rock formation, at which the elastic deformation of rocks occurs to irreversible.

This goal is achieved by the fact that when implementing a method for studying a core of terrigenous rocks, including saturating the core with fluid and placing it in a core holder, bringing the core and fluid in the core to reservoir conditions and determining the deformation properties of rocks with a stepwise decrease in the initial pressure in the core, deformation properties of rocks is determined with a cyclic stepwise decrease and the same increase in pressure in the core with an increase in this value at each cycle, the core deformation and the amount of pressure reduction in the core are fixed at which the core deformation, determined with a decrease in pressure, becomes different from the core deformation, determined at the same increase in pressure in the core, while the minimum such pressure decrease determines the pressure value, below which the deformation of the rocks of the core passes from elastic to irreversible, and the maximum, at which there is no deformation of the rocks at the stage of pressure increase, determines the pressure value, below which the deformation of rocks becomes only irreversible.

In FIG. Figure 1 shows the fundamental results of studying the change in the deformation properties of rocks with a change in the initial pore (reservoir) pressure in the core: line 1 - the dependence of the relative deformation of the studied core with a stepwise decrease in pore pressure on the value of this decrease, 2 - a similar dependence when implementing a cycle of increasing pore pressure by the same size.

The essence of the described invention is as follows.

The core is saturated with fluid and placed in the core holder of the installation for studying the properties of rocks. Further, by synchronously increasing the axial and lateral compressive pressure, as well as the pore pressure, initial conditions are created in the simulated rock formation. Pore pressure is reduced (while maintaining external pressures simulating reservoir conditions) and fixing the change in the linear size of the core - ΔL, then restoring the pore pressure to its original value and also fixing the value of ΔL. The study of the core is repeated many times with an increase in the magnitude of the decrease and the same increase in pore pressure in each cycle. Based on the results of such cyclic studies of the core, its relative deformation is determined - ΔL/L (L is the initial length of the core) and the dependence of the relative deformation of the core on the amount of pore pressure decrease in each cycle of decrease and increase in pore pressure is plotted, as shown in Fig. 1. The value of the formation pressure decrease is fixed, at which the relative deformation of the core, determined with a decrease in pore pressure, becomes different from the relative deformation, determined with the same increase in pore pressure. From the given example of studies (Fig. 1), such a value is fixed when the pore pressure decreases by 3. Such a decrease in pore pressure is the boundary of the transition from the elastic deformation of the core to irreversible. According to the value of the decrease in pore pressure, at which no deformation occurs in the core during the cycle of increasing the pore pressure, one judges the onset of only irreversible deformation in the core (in Fig. 1 - the value of the decrease in pore pressure 7). In the range of pore pressure change from 3 to 7, there is a transition from elastic deformation of the core to irreversible.

When implementing the described method, the nature of the deformation processes occurring in the simulated rock formation is determined, for example, in the reservoir of a developed oil and gas field or an operating UGS facility with a decrease in the initial formation pressure or its increase as a result of various technological operations. The range of creation and change of the initial pore pressure in the core is set taking into account the expected such changes in reservoir pressure in the considered field or UGS facility.

The use of this invention allows solving a number of important issues, for example:

- to determine the feasibility of constructing geodynamic test sites at underground gas storage facilities (UGS), since with proven irreversible deformations of rocks in the range of changes in the initial reservoir pressure in UGS collectors, there will be no change in the altitude position of the earth's surface;

- assign technological modes of operation of wells, in which drawdowns on the formation do not exceed the critical values of the transition from elastic deformation of rocks to irreversible, leading to the destruction of reservoirs in the bottomhole zone.

Claims (1)

A method for studying a core of terrigenous rocks, including saturating the core with fluid and placing it in a core holder, bringing the core and fluid in the core to reservoir conditions and determining the deformation properties of rocks with a stepwise decrease in the initial pressure in the core, characterized in that the deformation properties of rocks are determined at cyclic stepwise decrease and the same increase in pressure in the core with an increase in this value at each cycle, fix the deformation of the core and the amount of pressure decrease in the core, at which the deformation of the core, determined with a decrease in pressure, becomes different from the deformation of the core, determined with the same increase in pressure in the core, at the same time, according to the minimum such pressure decrease, the pressure value is determined, below which the deformation of the core rocks passes from elastic to irreversible, and according to the maximum, at which there is no deformation of the rocks at the stage of pressure increase, the pressure value is determined, below which the deformation of the rocks only becomes irreversible.

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